1. Field of the Invention
The present invention relates to an image reading apparatus, and more particularly relates to an apparatus for reading an image indicating features of a living body such as a fingerprint of a finger and another skin pattern, in order to authenticate a person.
2. Description of Related Art
Conventionally, as an image reading apparatus for authenticating a person by using a finger, an apparatus is known for reading a fingerprint that is a pattern of a skin of a fingertip. Various types of reading apparatus that uses an absolute value or a change value of a physical value such as light, electric field, pressure, capacitance and temperature, has been developed.
A method that uses a total reflection critical angle in a fiber optic plate (as disclosed in Japanese Patent No. 3045629: first conventional example) or a prism (as disclosed in U.S. Pat. No. 6,381,347: second conventional example) is widely used as a fingerprint input apparatus. FIG. 17 shows a conventional example that uses the total reflection critical angle of the prism. With reference to FIG. 17, a lens 106 and a 2-dimensional image sensor 107 are arranged in a direction perpendicular to a prism plane 109. A skin 104 of a finger is illustrated by enlarging the pattern of the skin. When a light 101 is inputted from an air portion having the refractive index of 1.0 where the skin is not in contact with the prism 105 in to the prism 105 having the refractive index of 1.4, the light has greatly refraction and is totally reflected on the prism plane 109, so that the light does not arrive at the 2-dimensional image sensor 107. However, a light 102 inputted into the prism 105 at a portion where the skin is in contact with the prism 105 never reaches the total reflection angle on the prism plane 109 because the refractive index of tats and oils or water on the skin or skin surface is near to that of prism glass so that a refraction angle on the prism plane 108 becomes small. Thus, the finger pattern is imaged on the 2-dimensional image sensor 107 by the lens 106. Thus, the pattern of the skin such as a fingerprint can be detected as a shadow pattern based on whether or not the concave and convex portions of the finger are brought into contact with the prism.
A conventional technique is proposed in which the optical system such as the prism and the lens is removed, although the 2-dimensional image sensor is used, in order to attain the miniaturization of an apparatus, and a finger is brought into contact with the 2-dimensional image sensor to detect a fingerprint image, as disclosed in Japanese Laid Open Patent Application (JP-P 2001-92951A: third conventional example). This conventional technique will be described below with reference to FIGS. 18A and 18B. The image reading apparatus shown in FIGS. 18A and 18B is provided with a 2-dimensional image sensor 2004 in which a plurality of photo sensors 2001 such as a double-gate type transistors are arranged in a matrix on a glass substrate 2002, and a insulating protection film 2003 having an optically transmissible property is coated on the entire surface; a transparent conductive film 2005 formed to have a predetermined pattern on the surface of the 2-dimensional image sensor 2004; and a planar light source 2007 which is placed on the rear of the 2-dimensional image sensor 2004 and emits a uniform light to the finger in contact with the top plane of the 2-dimensional image sensor 2004. Here, the transparent conductive film 2005 is composed of a pair of conductive patterns 2005a and 2005b, and at least one of them is grounded.
Also, both of the conductive patterns 2005a and 2005b are formed only on the mutual gap between the photo sensors 2001, in order to avoid the region immediately over the photo sensor 2001. The 2-dimensional image reading apparatus as configured above is operated as follows.
When a finger is placed to be in contact with a pair of conductive patterns 2005a and 2005b, the static electricity charged on the finger is discharged through any one of the conductive patterns 2005a and 2005b to the ground. Then, the operation for reading the fingerprint is started. That is, light is inputted to the finger through the 2-dimensional image sensor 2004 from the planar light source 2007, and is propagated while being scattered and reflected on the skin cortex of the finger. Then, a portion of the propagated light is inputted as excitation light into a photo sensor 2001 opposite to the convex (ridge) section of the fingerprint where there is no air layer whose refractive index is low on the boundary between the insulating protection film 2003 and the skin cortex of the finger. On the other hand, the other portion of the light is inputted into the photo sensor 2001 opposite to the concave (valley) section of the fingerprint where the air layer exists on the boundary between the insulating protection film 2003 and the skin cortex is suppressed. As a result, a pattern image is obtained in which the convex portion of the finger pattern serves as a bright region, and the concave portion serves as a dark region. In this way, in the image reading apparatus of FIGS. 18A and 18B, while the finger is brought into contact with the top plane of the 2-dimensional image sensor 2004, the fingerprint image is read. Thus, the transparent conductive film 2005 is made thin not to disturb the contact between the finger and the 2-dimensional image sensor 2004.
Similarly, the skin is brought into contact, so that a fingerprint image is obtained. However, in order to attain further miniaturization, other techniques re proposed in Japanese Laid Open Patent Applications (JP-A-Heisei 10-91769 and JP-P2001-155137A: fourth and fifth conventional examples). In such techniques, a quasi one-dimensional sensor of a pressure or temperature or capacitance type is used, and partial images of the fingerprint of a finger that is obtained by moving the finger in contact with the quasi one-dimensional sensor are linked to reconfigure the fingerprint image. In particular, methods that use the capacitance and the temperature are already available in a market. These methods contribute to the miniaturization and lower price of the apparatus.
Under such a situation, a non-contact fingerprint detecting apparatus is proposed as disclosed in Japanese Laid Open Patent Application (JP-P2003-85538A: a sixth conventional example). This conventional technique uses a phenomenon that when light is inputted into a finger, scattered inside the finger and emitted from the finger again, the light reflects the inner structure of the skin, so that the concave of the fingerprint serves as a bright region and the convex serves as the dark region. Thus, the dense/light image having the same shape as the fingerprint is obtained. According to this non-contact method, even in the finger whose skin is stripped due to dermatitis so that it is hard to read the fingerprint because contact of a skin separation portion is difficult in a method where the foregoing contact is assumed, the fingerprint image can be obtained if a portion of a structure inside the skin deriving a skin pattern is reserved. Also, in case of non-contact, it is difficult to receive the influence of the state change on the skin surface, such as a wet or dry state.
Also, a fingerprint input apparatus was proposed by the inventors of the present invention as disclosed in Japanese Patent No. 3150126 (a seventh conventional example). In this conventional apparatus, a fingerprint image is imaged by detecting the scattered emission light from the finger by a 2-dimensional image sensor located closely to the finger through a transparent protection cover made of glass. Thus, a concave portion of the fingerprint serves as a dark region and a convex serves as a bright region. This is hard to receive the influence of the external environment such as a wet or dry state of the finger, and the external disturbance light as compared with a sensor that uses pressure, temperature, capacitance and a total reflection critical angle. Also, as described in Japanese Laid Open Patent Application (JP-P2003-006627A; an eighth conventional example) proposed by the inventor of this application, the image of a high contrast can be obtained by optimally selecting the refractive index of the transparent protection cover.
In recent years, in conjunction with the advancement of information system, the leakage of person information and the spoofing of a different person in a transaction on a network become problematic. In order to prevent occurrence of those problems, an apparatus was developed which inputs a feature of a living body peculiar to a person and authenticates the person, instead of a method that easily allows the spoofing of the different person by stealing or furtively looking at a password or an authentication card. Also, the miniaturization of an information processing apparatus in a lower price represented by a portable phone have been advanced, and the apparatus for inputting the living body feature is also required to be miniaturized and cheapened. Moreover, since the personal authentication using the living body feature is applied for the settlement by using a credit card, the necessity of the higher precision of the living body feature input apparatus is increased more and more, in order to surely authenticate the person under any situation.
The property of the fingerprint that there is no same fingerprint from ancient times and it is never changed in one's life is verified in the police and justice fields, and the person authentication of a high precision is possible by using the fingerprint. However, in the conventional fingerprint input apparatus, it is difficult to obtain an excellent fingerprint image under a bad condition such as a wet or dry state of the finger, and skin peeling caused by dermatitis. Thus, although the fingerprint is not same between all people, it is hardly said to be able to be used for all people.
The fingerprint input method that uses a total reflection critical angle via the fiber optic plate (for example, the first conventional example) or the prism (for example, the second conventional example) is widely used for the personal authentication. However, as described in the related art, since the shade of the fingerprint is generated by the contact between the concave and convex sections of the skin and the prism, the image on the skin peeling portion is lost. Also, the use of the expensive large optical part obstructs the miniaturization and lower price of the apparatus. The image reading apparatus described in the third conventional example contributes to the miniaturization and the lower price, because the optical parts are removed. However, since the shade of the fingerprint is generated by the contact between the concave convex of the skin and the 2-dimensional image sensor plane, the image on the skin separation portion is lost.
With regard to the 2-dimensional sensor of the pressure, electric field or capacitance type, there are several actual use examples. Since the optical parts are removed, this contributes to the miniaturization and the lower price. However, any of them has a contact mechanism as the assumption, and the image on the skin peeling portion is lost. Also, as compared with the optical method, this type of apparatus is weak for the condition change such as the wet or dry state of the finger.
The technique that uses the quasi 1-dimensional sensor of a pressure, temperature, electric field or capacitance type and slides the finger in contact with the sensor and then reconfigures the image of the fingerprint of the finger (for example, the fourth and fifth conventional example) contributes to the further miniaturization and lower price of the apparatus. However, the image on a non-contact portion is lost. Thus, if the skin is partially stripped because of dermatitis, the fingerprint authentication, namely, the authentication based on the living body feature is difficult. Also, the method that uses the sensor of the 1-dimensional type and moves a reading target and reconfigures the image is already known in a facsimile and a copier. However, this technique has a problem where in order to miniaturize the apparatus, if the special mechanism for getting a speed of a direction in which the finger is moved is omitted, the image reconfiguration precision of the fingerprint is reduced.
As the technique for improving the decrease in the authentication precision caused by the peeling of the skin, a non-contact fingerprint detection apparatus is proposed in the sixth conventional example. According to this proposal, the emission light, which is inputted to the finger and scattered inside the finger and then emitted from the skin surface of the finger, reflects the inner structure of the skin. Thus, the dense/light shape corresponding to the fingerprint is observed. In this proposal, independently of the wet or dry state of epidermis and even when the epidermis horny layer is stripped and dropped because of dermatitis, if the structure of cutis serving as the origin of an epidermis pattern of the fingerprint is reserved, the fingerprint image is obtained. However, in case of the fingerprint detecting apparatus described in the sixth conventional example, a fixing frame for fixing the finger is required and an image forming optical system is also required, which obstructs the operability and miniaturization of the apparatus. Also, the finger and the image forming system are greatly separated. Thus, even if the inner structure of the finger causes a light quantity emitted from the skin surface to be changed, it is scattered on the skin surface, and the event, which is estimated based on a adverse influence caused due to a spread resulting from the distance of the image forming system, resulting in a problem that the fingerprint image of the excellent contrast is not obtained in the portion where the skin is actually stripped.
On the contrary, in the fingerprint authenticating apparatus (the seventh conventional example) invented by this inventor, the emission light that is emitted from the skin surface after scattered inside the finger is imaged by the 2-dimensional image sensor located closely to the finger, and the fingerprint image is obtained. Then, the miniaturization and lower price of the apparatus are attained. Also, in the technique for reading the scattered emission light from the finger, since the light is once inputted to the inside of the finger, the structure inside the finger is obviously reflected. Thus, in the fingerprint input apparatus according to the seventh conventional example by this inventor, the optical image forming system is removed, thereby attaining some small fingerprint detecting apparatus, and as the phenomenon in the non-contact portion where the skin is stripped, the image in which the inner structure of the skin of the finger is reflected, as pointed out in the sixth conventional example.
On the other hand, the fact that the fingerprint image through the scattered emission light from the finger greatly depends on the boundary state between the skin and the sensor protecting film is clarified by the eighth conventional example related to the proposal of this inventor. That is, the eighth conventional example describes that a refractive index of a transparent cover existing between the fingerprint and the 2-dimensional image sensor placed closely thereto is selected so as to increase the contrast between the bright region corresponding to the convex of the fingerprint in contact with the transparent cover and the dark region corresponding to the concave that is not contact. However, in case of such selection, the influence of the reflection and refraction of the boundary becomes strong which decreases the component reflecting the skin structure. Thus, this has a problem where it is hard to obtain the contrast of the fingerprint image in which the skin structure originally appearing in the skin separation portion is reflected. This problem is especially severe in case where a dynamic range is not widely set. If the non-contact state is kept, the influence of the boundary is removed. However, the configuration for using the fixing frame for the finger and the image forming optical system as proposed in the sixth conventional example brings about the foregoing problem.